This invention relates to nuclear reactors. It has particular relationship to the structure of the core and core support of a nuclear reactor. The core of a nuclear reactor includes a plurality of fuel assemblies. Each assembly is an elongated member of generally polygonal transverse cross section. Typically, the cross section may be square or rectangular, the fuel assembly having the form of a square or rectangular parallelopiped. The cross section may also be hexagonal. In forming the core, the fuel assemblies are mounted or nested side-by-side in an array. It is essential that the fuel assemblies be precisely positioned and aligned and firmly supported in such a way that the positioning and alignment is maintained under the condiserable forces exerted by the coolant during operation and notwithstanding the extensive temperature variations which occur during operation.
A description of the typical prior art is presented in Anthony, U.S. Pat. No. 4,192,716. In nuclear reactors prior to Anthony, pins were provided in the core support plate and holes in each fuel assembly or pins were provided in the each fuel assembly and holes in the core support plate. In either structure, the fuel assemblies were mounted with each pin in a fuel assembly on the support plate in a hole in the support plate or fuel assembly as the case may be. Anthony describes some of the difficulties and drawbacks of this structure. Difficulty of manufacture and cost are important factors. There are 192 fuel assemblies in a typical reactor and two alignment pins in, or for, each assembly. To provide each of the 384 pins, the pin-receiving member must be drilled and tapped and threaded pins must be screwed into the tapped holes. An important difficulty not mentioned by Anthony is that the pins do not position and align the fuel assemblies as effectively as is necessary to assure that they may maintain their position and alignment.
Anthony proposes to provide pins in the core support plate and depressions in the corners of the lower end fittings of each fuel assembly, each corner engaging a quarter of a pin, where the fuel assemblies are of square transverse cross section. The disadvantages of this structure is that the pins may be bent or deformed as the assemblies are lowered into engagement with them. To repair such a bent or deformed pin, it is necessary that the core plate support and the core barrel which it carries be removed from the pressure vessel, a formidable task particularly if the damage to a pin occurs after a number of fuel assemblies have been engaged with the core support plate.
It is an object of this invention to overcome the above-described difficulties and disadvantages of the prior art and to provide a positioning and alignment structure for the fuel assemblies of a nuclear reactor which shall be readily manufactured at lower cost than prior art structures and which shall not require removal of the core support plate from the pressure vessel in the event of damage during installation of fuel assemblies.